Aragonite−Hydroxyapatite Conversion in Gastropod (Abalone) Nacre

From studies of geological and coral skeleton aragonite, it was previously proposed that aragonite converts hydrothermally to hydroxyapatite (HA) in phosphate solution by a solid-state topotactic ion-exchange reaction in which the HA retains the orientation of the original aragonite. The present investigation of this reaction in gastropod shell nacre suggests instead that conversion for this biomineral proceeds via dissolution-recrystallization, with the preferred orientation of the product determined by the flux direction of the solution through the material. Conversions of various CaCO 3 biominerals in aqueous (NH 4 ) 2 HPO 4 at 140−260 °C show that for gastropod nacre alone, sample form and internal ultrastructure are largely preserved and a unique preferred orientation arises: with reaction time, HA nanocrystallites are increasingly oriented with their c -axes perpendicular to the c -axis of the original aragonite. In contrast, other CaCO 3 biominerals either disintegrate to HA powder or contain HA crystallites with random orientation or the orientation of the original aragonite. We suggest that the organic matrix of gastropod nacre channels the reaction solution through the material vertically between stacks of aragonite tablets so that sample form and internal ultrastructure are preserved and the HA predominantly adopts a perpendicular c -axis orientation. Other properties of HA-converted gastropod nacre are also discussed.